1. Field of the Invention
This invention relates to optical recording media in which digital data are recorded by a laser beam acting upon an absorptive surface to change the reflectivity. More particularly, it relates to a recording medium in which a metal film overlies a thermoplastic substrate and in which the film has increased absorption because of surface discontinuities in the substrate.
2. Description of Related Art
A number of earlier patents of the present inventor, such as U.S. Pat. No. 4,811,331, describe a recording medium formed of a thermoplastic substrate having an array of parallel rows of uniformly spaced microscopic individually-identifiable optically-alterable recording spots, called micromirrors. Each micromirror is capable of storing one bit of digital information. The surface of each micromirror is coated with a reflective material such as a composite of gold and silicon dioxide. To record data, a laser beam is focused successively on the micromirrors leaving each micromirror unchanged (to indicate, say, a digital zero) or with a detectably reduced reflectivity (to indicate, say, a digital one). If the reflectivity is to remain unchanged, the laser beam is adjusted to have enough intensity to identify the presence of the micromirror but insufficient energy to materially change the reflectivity of the micromirror. If the reflectivity of the micromirror is to be reduced, the intensity of the laser beam is increased so that a particular micromirror is destroyed to the extent that it has reduced reflectivity. The laser beam does not reduce the reflectivity of any of the micromirrors to zero, but allows enough reflectivity to remain that the presence of the micromirror is readily identified by the reading laser that distinguishes between micromirrors of different reflectivity.
Such discrete micromirrors serve as counting fixtures from which the position of the recording and reading beams can be continually determined. This is particularly important when the medium is used in a scanning system in which the scanning velocity of the laser beam is not constant. For example, when scanning an optical card along successive rows of micromirrors, the reading and writing mechanisms are much simplified if it is not necessary to maintain a constant scanning velocity during reading and writing. For example, at the end of each row of micromirrors, it is necessary to reverse the direction of the scan, either by reversing the direction of the beam or the movement of the optical card. One of the most efficient ways to do this is for the scanning beam to move, relative to the card, with a generally sinusoidal motion. This is a feasible alternative with the use of the uniformly spaced micromirrors.
The coating material used on such optical media must be applied as a uniform extremely thin layer and the formulation is critical. These factors add significantly to the cost of an optical recording card. Many metals or metallic compounds have the necessary stability for long term storage, but do not have the sufficient absorption for recording with low-cost diode lasers. It is desirable to use pre-formed recording elements, or at least a recording track, that must have sufficient reflection for detection of its position, but must absorb sufficient energy that a beam of moderate power can materially reduce the reflectivity of the film. For example, gold in a uniform thin film has the necessary stability and is readily detectable by a laser beam, but may have as much as 80% reflectivity and 15% transmission with only about 5% or less of the energy being absorbed by the film. Recording on such a film requires excessively high power to alter the reflectivity. If SiO.sub.2 is combined in a gold film, the absorption is increased to about 20%. Such a film permits the use of an 8 milliwatt laser beam with a 300 nanosecond exposure. An exposure time of only 100 nanoseconds or less is much to be desired because of the potentially higher recording speed. A more or less ideal medium would have an absorption of about 50% and a reflectivity of about 50%, preferably with little or no transmission.
Tellurium and tellurium alloys have been widely used in optical recording media because of their low melting points, but have a history of serious problems because of their chemical instability and toxicity. Sealing the tellurium based recording layer between sheets of glass has been one approach to solving these problems but has drawbacks related to media cost and potential for breakage.
Craighead in U.S. Pat. No. 4,422,159 describes a medium formed of material such as germanium or silicon that is etched to produce a structure having a multiplicity of columnar or conical features spaced less than one wavelength apart. The resulting medium appears black because almost all of the light is internally reflected within the germanium or silicon columns and is not reflected back toward the source. The Craighead medium has high absorption and substantially no reflectivity. After treatment with a laser beam the medium exhibits increased reflectivity where the surface has been melted to reshape the columns into a smooth reflective surface. Such a medium permits high recording sensitivity because of the absorption of almost all of the light but does not permit the identification of recording tracks prior to recording. Because recording requires melting the inorganic crystallites comprising the film, a high temperature is required to melt the crystallites and change the reflectivity of the film. The medium described in the Craighead patent cannot tolerate a nonuniform scanning speed.